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Spiral chip inductors are planar coils built as thin-film metallization on a ceramic substrate, used as surface-mount or bare-die inductors in RF and microwave circuits. Choose them when you need a defined inductance with high Q and a known self-resonant frequency in a small, repeatable footprint. The main decision is which variant fits your circuit: a high-Q single-value part, a multi-tap tuning part, a flip-chip part, or a chip-scale gold stud bump part. US Microwaves lists all four families, with values as low as 1 nH to 10 nH in its LX3000Q20NH line.

How a spiral chip inductor works

A spiral inductor is a flat coil: current enters one end, spirals inward, and exits at the center or through an underpass. Because the coil is planar, inductance comes mainly from the number of turns, the coil diameter, and the trace width and spacing. Thin-film processing keeps those dimensions tight, so inductance is repeatable from part to part.

Three parameters drive most selection decisions:

• Inductance (L): set by geometry; the LX3000Q20NH family covers 1 nH to 10 nH.
• Q factor: the ratio of stored to lost energy. Higher Q means a sharper resonance and lower loss in matching and tuning networks. US Microwaves labels the LX5000Q20, LX3000Q20, and LX2500Q20 families as high-Q.
• Self-resonant frequency (SRF): the frequency where the coil's own capacitance resonates with its inductance. Above SRF the part behaves capacitively, so the inductor is only useful below it. US Microwaves describes these parts as high Q and high self-resonance.

A practical example: in a 5 GHz matching network, an inductor whose SRF sits near 5 GHz will look capacitive at the design frequency. You need a part whose SRF is comfortably above the band you are matching, which usually means a smaller inductance or a geometry with lower parasitic capacitance.

Variant comparison

US Microwaves organizes its spiral chip inductors into families that differ in mounting and tuning approach. The table below compares them on the dimensions that matter for selection.

The naming pattern is consistent: the number (5000, 3000, 2500) identifies the geometry family, and the suffix identifies the mounting or tuning option. The LXT prefix marks multi-tap tuning parts; FC marks flip chip; CSP marks chip scale with gold stud bumps.

Matching the part to the circuit need

Start from what the circuit must do, then pick the family.

Fixed impedance matching or resonant tanks. Use the high-Q single-value families (LX5000Q20, LX3000Q20, LX2500Q20). Higher Q narrows the resonance and lowers loss, which matters in filters and narrowband matching. For values in the 1 nH to 10 nH range, the LX3000Q20NH line is the listed option.

Circuits that need trimming after assembly. Use the multi-tap 1/4-turn tuning families (LXT5000Q20, LXT3000Q20, LXT2500Q20). The taps let you select a turn count to adjust inductance on the board, which is useful when substrate tolerances or component parasitics shift the design away from simulation.

Space-constrained or die-level assemblies. Use flip-chip (LXT…Q20FC) or chip-scale gold stud bump (LX…Q20CSP, LXT…Q20CSP) parts. Flip-chip mounting shortens the interconnect, which reduces added parasitics; gold stud bumps suit direct die attach in dense layouts.

Very small inductance values. The LX3000Q20NH family is the explicit 1 nH to 10 nH high-Q option in the listing.

Selection and layout checklist

Before committing to a part, confirm these points:

1. SRF margin. Check that the self-resonant frequency is well above your highest operating frequency. A part that is "high self-resonance" in general terms may still be too close for a millimeter-wave design.
2. Q at your frequency. Q is frequency-dependent; a high-Q family is high-Q over its intended band, not everywhere.
3. Mounting style. Match the part to your assembly process: surface-mount chip, flip chip, or gold stud bump. The suffix tells you which.
4. Tuning need. If the design may need adjustment, the multi-tap 1/4-turn parts give you that option without a board respin.
5. Value range. Confirm the inductance you need falls within the family's range; the LX3000Q20NH line is the listed small-value option.

Getting parts and pricing

US Microwaves provides contact, quote request, cross-reference, inventory check, user login, and online order entry on its site, and lists an online quotation path for custom thin-film circuits. The source material does not state prices, minimum order quantities, or whether ordering requires an account, so confirm those directly with the manufacturer before planning a build. For custom thin-film work, the site also documents layout design rules and application notes that are worth reviewing alongside the inductor datasheets.

Thin film microwave circuits are microwave integrated circuits (MICs) built by depositing and patterning thin conductive and resistive films on a ceramic substrate, then adding chip components and transmission lines on top. They suit designs that need tight tolerances, high Q, high self-resonance, and repeatable performance at microwave frequencies. US Microwaves, a manufacturer of this technology, lists the full build chain on its site: mask manufacturing, layout design rules, thin film ceramic substrate specification, 50-ohm microstrip lines, bonding islands and stand-offs, aluminum nitride submounts and heat sinks, and custom thin film circuits with online quotation.

What a thin film microwave circuit is

A thin film MIC is a hybrid circuit: passive metal patterns (conductors, resistors) are formed on a ceramic wafer by thin film deposition and photolithography, and discrete chip components are attached to complete the function. The substrate is part of the circuit design, not just a carrier — its dielectric properties set the impedance of microstrip lines, and its thermal properties set power handling.

This differs from a monolithic microwave IC (MMIC), where transistors and passives are fabricated in one semiconductor process. A thin film MIC combines high-quality passive elements with separately chosen active devices, which is useful when the passive performance (Q, tolerance, thermal path) matters more than maximum integration density.

What goes into the circuit

US Microwaves organizes its thin film products into three component families that populate these circuits:

The circuit itself is completed with 50-ohm microstrip transmission lines, bonding islands, and stand-offs, all defined by the thin film layout.

How the circuit is built

The manufacturer's published process flow gives the practical sequence:

1. Layout and design rules. Start from the thin film MIC layout design rules and the thin film ceramic substrate specification. These define minimum features, line widths for 50-ohm microstrip, and bonding pad geometry. Getting the substrate spec right first avoids re-spins.
2. Mask manufacturing. Photomasks are generated from the layout; the site lists mask manufacturing as an in-house capability.
3. Thin film deposition and patterning. Conductor and resistor films are deposited on the ceramic substrate and patterned through photolithography. Coating services are also offered separately.
4. Substrate and carrier selection. Alumina (Al2O3) is the standard substrate; aluminum nitride (AlN) is used where heat spreading matters. Gold-plated Kovar carriers, submounts, and heat sinks provide the mechanical and thermal interface.
5. Component attach and interconnect. Chip inductors, capacitors, and resistors are bonded to the defined islands; flip chip and chip scale variants change the attach method and parasitics.
6. Verification. Check impedance of microstrip sections against the 50-ohm spec, verify component values and tolerances, and confirm thermal path for power resistors before committing to volume.

The application notes and technology overview on the manufacturer's site are the reference for steps 1 and 3; the design rules document is the one to read before drawing a layout.

Where thin film MICs fit — and where they don't

Choose thin film when:

• Passive Q and self-resonance drive performance. Spiral inductors offered as high-Q, high-self-resonance parts exist precisely because printed or thin-film-on-silicon alternatives lose Q at higher frequencies.
• Tolerance and repeatability matter. Thin film lithography gives tighter control than thick film printing.
• Thermal management is a design constraint. AlN substrates, submounts, and heat sinks are part of the standard offering, and power resistor families are specified on AlN and BeO.
• You need a custom passive network — matching, combining, filtering — around active devices such as RF bipolars or LDMOS transistors, which the same manufacturer also lists.

Consider alternatives when:

• Maximum integration and lowest size/weight are the priority — a MMIC may be the better fit.
• Cost at high volume dominates and looser tolerances are acceptable — thick film or PCB-based approaches may compete.
• Your frequency is low enough that laminate transmission lines and commercial SMT passives meet the spec.

Getting a custom circuit quoted

The manufacturer provides an online quotation path for custom thin film circuits, alongside design services for thin film layouts. A practical request includes: substrate material and thickness, layer stack and metal system, minimum feature sizes from the design rules, component list with values and tolerances, carrier or submount requirements, and expected power dissipation per resistor. Cross-reference and inventory check tools are also listed for part selection. Pricing and lead times are not published on the site, so they must come from the quotation.

LDMOS (laterally diffused MOSFET) is a silicon power MOSFET structure in which the channel is formed by lateral diffusion of dopants under the gate, with a lightly doped drain drift region that sustains high voltage while keeping resistance low. That combination—high breakdown voltage, low on-resistance, and a planar structure with the source connected to the substrate—is why LDMOS became the dominant RF power device for base-station and broadcast amplifiers in the roughly 1–4 GHz range. Choose LDMOS when you need high output power, ruggedness under mismatch, and good thermal spreading through a substrate-connected source; look elsewhere (GaN, SiC, or RF bipolars) when you need very high frequency, very high power density, or specific linearity behavior.

What the device structure does for RF

The "lateral diffusion" refers to how the channel length is set: dopants diffuse sideways under the gate rather than being defined purely by lithography. This gives short, reproducible channels without advanced lithography. The lightly doped drain (LDD) drift region between channel and drain contact drops most of the drain voltage, so the device can run at 28–50 V supplies—much higher than typical RF bipolars—which directly raises achievable output power per unit of current.

The source is typically tied to the bottom of the die (source-to-substrate connection). In RF packages this means the source inductance is minimized and heat flows straight into the package flange and heatsink. That is a structural advantage over bipolar devices, where the collector is usually the backside contact and the emitter is on top.

LDMOS vs. RF bipolars and other microwave transistors

The practical takeaway: LDMOS is a workhorse for high-power, moderate-frequency amplification where ruggedness and thermal handling matter. RF bipolars remain relevant where their gain and linearity characteristics are already designed in, and GaN is the choice when frequency or power density pushes beyond what silicon LDMOS delivers.

Where LDMOS is commonly used

• Cellular base-station power amplifiers, especially in the 1–3 GHz bands
• Broadcast transmitters (FM, DTV) where high power and efficiency dominate
• Industrial, scientific, and medical RF generators
• Radar and avionics transmitters in the lower microwave range

In each case the common thread is a requirement for substantial continuous output power with a manageable thermal design.

Selection parameters that actually decide the part

1. Frequency range and gain. Check gain at your exact operating frequency, not just the datasheet's peak. Gain falls with frequency, so margin at the band edge matters.
2. Output power and compression point. Specify P1dB and saturated power for your modulation; for multi-carrier signals, back off from saturation and check intermodulation (IMD) performance at that back-off.
3. Efficiency. Drain efficiency at your operating point drives heatsink sizing and supply design.
4. Supply voltage. LDMOS devices are designed around specific drain voltages; matching your rail to the device's intended operating point keeps efficiency and linearity in the specified range.
5. Package and thermal resistance. The junction-to-case thermal resistance plus your heatsink determines junction temperature. Power flanged packages and heat-sink sub-mounts are common in microwave assemblies.
6. Matching and bias. LDMOS input impedance is largely capacitive and low at high power, so input matching networks are essential; gate bias networks must be decoupled carefully to avoid low-frequency instability.

Integration concerns in thin-film MIC assemblies

In a thin-film microwave integrated circuit environment, LDMOS devices are typically mounted as discrete die or packaged parts alongside thin-film matching networks, chip capacitors, spiral inductors, and resistors. Practical points:

• Biasing: provide a clean, well-decoupled gate bias; sequence gate and drain supplies so the device is never operated without proper gate voltage.
• Matching: design input and output matching for the actual operating band and power level; verify with load-pull or measured S-parameters rather than relying on small-signal data alone.
• Heat sinking: use flanged packages, aluminum nitride sub-mounts, or heat-sink carriers matched to the thermal load; the source-to-substrate path is only as good as the mounting interface.
• Stability: check for low-frequency oscillation with the full bias network and realistic source/load impedances.

US Microwaves lists LDMOS among its RF power transistor offerings alongside RF bipolars, thin-film MIC circuits, spiral chip inductors, ceramic and MOS/MNOS capacitors, and chip resistors—useful when a single assembly needs the transistor plus its passive matching and bias components sourced together. For current part numbers, frequency coverage, and pricing, request a quote or cross-reference through the site rather than assuming catalog availability.

RF bipolar transistors are NPN or PNP junction devices optimized for signal amplification or power generation at radio and microwave frequencies, rather than for switching or low-frequency analog work. Choose them when you need high gain per stage, low phase noise, or a proven, well-characterized die for a hybrid MIC; choose LDMOS or other FETs when you need higher breakdown voltage, simpler biasing, or high power at lower frequencies. The selection criteria below apply to both packaged parts and bare-die devices of the kind US Microwaves lists under its RF power transistors and bare-die products.

What "RF bipolar" means in practice

An RF bipolar transistor is a BJT whose geometry, doping profile, and package (or die layout) are designed so that current gain and power gain hold up into the hundreds of MHz to several GHz. Compared with a general-purpose BJT, the RF version trades ruggedness and low-frequency beta for:

• Higher fT/fmax — thinner base, smaller emitter stripes, lower parasitics.
• Controlled input/output impedance — often specified as S-parameters at a reference plane.
• Thermal paths designed for the die — critical for power devices and for bare-die MIC assembly.

In a thin-film MIC flow, the bipolar is typically a bare die mounted on a carrier or submount, with matching networks built from spiral chip inductors, ceramic or MOS/MNOS capacitors, and thin-film resistors on the same substrate.

RF bipolar vs. LDMOS and other RF power devices

The practical rule: use bipolars where gain, noise, and phase noise dominate the budget; use LDMOS where power and ruggedness dominate.

Key selection parameters

1. Frequency range and gain. Work from the vendor's S-parameters or gain-vs-frequency curve at your bias point, not from a single "typical" number. Check gain at the top of your band, where it is lowest.
2. Output power and compression point. Select for P1dB and saturated power with margin over the required output, then confirm the gain at that drive level.
3. Breakdown voltage and safe operating area. Verify VCEO and VCBO against your supply and expected load mismatch. A mismatched load can double the voltage swing at the collector.
4. Bias current and thermal resistance. Get Rth(j-c) or Rth(j-ambient) for the die or package, compute junction temperature at worst-case ambient, and derate. For bare die, include the die-attach and carrier in the thermal path.
5. Noise figure (for LNAs) and phase noise (for oscillators). These are bias-dependent; specify the bias point at which the number was measured.
6. Input/output impedance and matching. Confirm the reference plane and whether the data is for the die or the packaged part. Die-level data is what you need for a MIC.

Matching and biasing in amplifier and MIC designs

Matching

• Input match usually needs a conjugate match at the design frequency; for broadband LNAs, trade gain flatness and noise figure against return loss.
• Output match should be designed for the load at the collector, with harmonic termination if efficiency matters.
• Bias network as part of the match. The bias tee or collector feed is not neutral — its inductance and bypass capacitors shift the match. Simulate the bias network with the device, not after it.
• Use the vendor's capacitor and inductor families consistently. For example, US Microwaves lists spiral chip inductors (LX/LXT series, including high-Q and multi-tap tuning types) and silicon MIS/MNOS/MOS, ceramic, and quartz capacitors, plus thin-film resistors. Choosing parts from one characterized family reduces surprises between simulation and hardware.

Biasing

• Current-source or resistor bias with emitter degeneration. Emitter resistance stabilizes bias against beta spread and temperature; too much costs gain and adds noise.
• Temperature compensation. Use a diode or transistor sensor in the bias network, or active bias, to hold collector current over temperature.
• Sequencing and protection. Apply bias in a controlled order, limit current during turn-on, and protect against load mismatch and ESD. Bipolars are less forgiving than LDMOS here.

Where bipolars are the better choice

• Low-noise amplifier first stages where noise figure and gain per stage matter more than raw power.
• Oscillators and low-phase-noise sources.
• Driver and medium-power stages in hybrid MICs where a compact, well-modeled die is preferred.
• Designs already built around a characterized bipolar die and matching library, where re-qualifying an LDMOS would add cost and schedule.

Where LDMOS or FETs usually win: high-power output stages, applications with severe load mismatch, and designs where simple voltage bias and ruggedness outweigh gain-per-stage.

Common failure modes and design checks

• Thermal runaway. Check junction temperature at maximum ambient and worst-case bias; verify emitter resistance or active bias limits current rise.
• Load mismatch overvoltage. Evaluate collector voltage at the worst expected VSWR; add output protection or derate.
• Bias network instability. Bipolars can oscillate with inductive bias feeds or inadequate bypassing; check low-frequency stability, not just in-band.
• Beta spread and lot variation. Design bias for the specified min/max beta, and verify gain across that spread.
• Die-attach and wire-bond parasitics. For bare die, include bond-wire inductance and attach thermal resistance in the model; these often dominate at microwave frequencies.
• Reference-plane errors. Confirm whether S-parameters are de-embedded to the die or include package and fixture; a mismatch here invalidates the match.

Practical next step

Start from the stage requirements — frequency band, gain, output power, noise or phase-noise target, and thermal budget — then shortlist bipolar parts whose data covers that band at your bias point. Request S-parameters and thermal data at the die level, build the bias network into the matching simulation, and verify junction temperature and mismatch tolerance before layout. US Microwaves' site provides product listings and a quote/cross-reference path for its RF power transistors and thin-film component families; use those listings to confirm which parts and matching components are available for your MIC.

Ceramic capacitors are the workhorse passive in thin-film MIC and RF/microwave assemblies, used mainly for DC blocking (coupling), RF bypass, impedance matching, and tuning. Choose them by starting from the operating frequency and the impedance the part must present there: a capacitance value alone is not a selection. The value must be paired with a dielectric whose stability and loss suit the circuit, a package size and termination that fit the assembly method, and a self-resonant frequency safely away from the band of use. US Microwaves lists ceramic chip capacitors alongside its thin-film MIC, spiral chip inductor, and ceramic/silicon resistor lines, positioning them as part of a bare-die and chip-component microwave portfolio rather than as general-purpose board-level parts.

Where ceramic capacitors sit in a microwave circuit

In MIC and thin-film microwave work, ceramic capacitors typically appear in four roles:

• DC blocking / coupling: passes RF while isolating bias nodes. Here the capacitor must look like a low impedance across the band, so value and SRF matter more than absolute tolerance.
• RF bypass: shunts RF to ground at bias feeds and supply nodes; low ESR and a low-impedance path at the operating frequency are the priorities.
• Matching and tuning: small values shift phase or resonate with inductors; tolerance and temperature stability directly affect whether the match holds over temperature.
• Filter and resonant elements: the capacitor becomes part of a tuned structure, where Q and SRF define insertion loss and spurious behavior.

Because these roles stress different parameters, the same "good" capacitor can be right for bypass and wrong for a tuned match.

The parameters that actually decide the choice

Dielectric type: the stability-versus-density trade

Stable, low-loss dielectrics (the NP0/C0G family) hold capacitance over temperature and voltage and are the default for matching, tuning, and narrowband filters. High-K ceramics pack much more capacitance into a small chip, which suits bypass and bulk decoupling, but their capacitance drifts with temperature, voltage, and time, and their losses are higher. In a tuned microwave node that drift detunes the circuit; in a bias bypass it is usually harmless. The decision rule: if the circuit's performance depends on the capacitor's value, use a stable dielectric; if the capacitor only needs to be "big enough" at RF, a high-K part can be acceptable.

SRF: the parameter most often ignored

Every capacitor is a series RLC. Below SRF it looks capacitive; at SRF it is resistive; above it, inductive. A 100 pF part chosen for bypass at a few gigahertz may already be past resonance and no longer shunting. Two practical consequences:

• For coupling and bypass, verify the impedance at the highest operating frequency, not the center.
• For broadband bypass, parallel different values so one part's inductive region is covered by another's capacitive region, and check that the parallel combination does not create an anti-resonance in band.

Size, termination, and assembly

At microwave frequencies the package is part of the circuit. Smaller chips have lower parasitic inductance and higher SRF, which is why small-case ceramic chips dominate high-frequency bypass and coupling. Termination style must match the assembly: solderable terminations for surface-mount and epoxy attach, and bondable metallization where wire or ribbon bonding is used. US Microwaves' product structure separates chip components (capacitors, inductors, resistors) from bare-die and thin-film circuit products, so the capacitor's termination and mounting must be chosen together with the interconnect method of the MIC, not after it.

For example, a thin-film matching network that will be ribbon-bonded needs capacitors with bondable terminations and a footprint that fits the layout rules of the thin-film process; a solder-attached bypass on a carrier can use a standard solderable chip.

A practical selection sequence

1. Define the role and the band. Coupling, bypass, match, or tune; note the lowest and highest frequencies.
2. Compute the target impedance at the band edges and convert it to a capacitance range.
3. Pick the dielectric from the stability requirement: stable (NP0/C0G-type) for match/tune/filter, high-K only for bypass where drift is tolerable.
4. Check SRF against the highest frequency; downsize the case if SRF is too low.
5. Confirm voltage margin for peak RF plus DC bias.
6. Match termination and footprint to the assembly method (solder, epoxy, or bond).
7. Verify in the actual circuit: measure insertion loss or return loss with the chosen part, and sweep temperature if the node is tuned.

Common failure points and how to catch them

• Using a part above its SRF. Symptom: bypass that works at low frequency but not at the band of interest. Fix: check impedance at the top of the band, not the nominal value.
• High-K dielectric in a tuned node. Symptom: match or filter response drifts with temperature or power. Fix: switch to a stable dielectric, or accept and characterize the drift.
• Ignoring parallel anti-resonance. Symptom: a notch or peak in the bypass response between two parallel values. Fix: simulate or measure the parallel combination, not each part alone.
• Termination mismatch. Symptom: poor bond or solder joint, or excess parasitics at the launch. Fix: select termination and footprint with the assembly method from the start.
• Voltage derating overlooked. Symptom: value loss or failure under high RF swing. Fix: derate for peak RF plus DC, and confirm with the manufacturer's data.

Where to go next

US Microwaves publishes ceramic chip capacitors as one line within its thin-film microwave components, alongside spiral chip inductors, ceramic and silicon resistors, and custom thin-film MIC services. For a specific design, use the site's quote request and cross-reference/inventory tools to confirm the exact part, termination, and availability, and pair the capacitor selection with the inductor and resistor choices so the parasitics and assembly method are consistent across the network.